3. burning: This is also the most intuitive method widely adopted by people in the industry. Pure cotton yarn, at one point, the flame is yellow, the smell of burning is like burning paper, after burning the edge is soft, leaving a few gray-black flocculent ashes; polyester and cotton yarn near the flame will shrink and then melt, black smoke, emitting a bad aromatic smell, burning after the edge hardened, ashes for Dark brown lump, but it can be twisted.